style: avoid testing against ourselves in lookup_for_rules.
style: avoid testing against ourselves in lookup_for_rules.

There's a path in finish_restyle, where we can recascade a style for a given
element, in get_after_change_style.

I haven't been able to construct a test-case for this, but this is the only
coherent story I have for crashes like:

  https://crash-stats.mozilla.com/report/index/bcdfe629-ca1f-4e4d-aa17-27f890170917#tab-details

ConnorGBrewster/servo,notriddle/servo,splav/servo,splav/servo,dati91/servo,cbrewster/servo,peterjoel/servo,thiagopnts/servo,szeged/servo,splav/servo,nnethercote/servo,canaltinova/servo,KiChjang/servo,ConnorGBrewster/servo,larsbergstrom/servo,KiChjang/servo,dsandeephegde/servo,DominoTree/servo,paulrouget/servo,saneyuki/servo,KiChjang/servo,paulrouget/servo,emilio/servo,paulrouget/servo,eddyb/servo,jimberlage/servo,saneyuki/servo,dati91/servo,larsbergstrom/servo,upsuper/servo,fiji-flo/servo,saneyuki/servo,mattnenterprise/servo,pyfisch/servo,fiji-flo/servo,avadacatavra/servo,anthgur/servo,splav/servo,SimonSapin/servo,jimberlage/servo,emilio/servo,rnestler/servo,pyfisch/servo,nnethercote/servo,paulrouget/servo,dati91/servo,notriddle/servo,notriddle/servo,upsuper/servo,anthgur/servo,KiChjang/servo,nnethercote/servo,peterjoel/servo,fiji-flo/servo,dsandeephegde/servo,thiagopnts/servo,CJ8664/servo,paulrouget/servo,splav/servo,peterjoel/servo,fiji-flo/servo,notriddle/servo,SimonSapin/servo,peterjoel/servo,emilio/servo,ConnorGBrewster/servo,avadacatavra/servo,larsbergstrom/servo,CJ8664/servo,eddyb/servo,emilio/servo,DominoTree/servo,nnethercote/servo,rnestler/servo,eddyb/servo,upsuper/servo,canaltinova/servo,canaltinova/servo,notriddle/servo,sadmansk/servo,splav/servo,canaltinova/servo,mattnenterprise/servo,SimonSapin/servo,cbrewster/servo,CJ8664/servo,mattnenterprise/servo,mbrubeck/servo,splav/servo,nnethercote/servo,sadmansk/servo,KiChjang/servo,SimonSapin/servo,larsbergstrom/servo,rnestler/servo,szeged/servo,peterjoel/servo,paulrouget/servo,dsandeephegde/servo,pyfisch/servo,saneyuki/servo,paulrouget/servo,SimonSapin/servo,CJ8664/servo,emilio/servo,larsbergstrom/servo,rnestler/servo,mbrubeck/servo,paulrouget/servo,cbrewster/servo,KiChjang/servo,CJ8664/servo,eddyb/servo,jimberlage/servo,avadacatavra/servo,upsuper/servo,ConnorGBrewster/servo,upsuper/servo,paulrouget/servo,thiagopnts/servo,splav/servo,notriddle/servo,SimonSapin/servo,dati91/servo,mattnenterprise/servo,upsuper/servo,canaltinova/servo,mbrubeck/servo,cbrewster/servo,szeged/servo,eddyb/servo,anthgur/servo,fiji-flo/servo,thiagopnts/servo,avadacatavra/servo,sadmansk/servo,notriddle/servo,KiChjang/servo,thiagopnts/servo,saneyuki/servo,avadacatavra/servo,jimberlage/servo,ConnorGBrewster/servo,sadmansk/servo,emilio/servo,emilio/servo,dati91/servo,mattnenterprise/servo,fiji-flo/servo,peterjoel/servo,upsuper/servo,peterjoel/servo,paulrouget/servo,sadmansk/servo,cbrewster/servo,peterjoel/servo,dati91/servo,szeged/servo,canaltinova/servo,peterjoel/servo,nnethercote/servo,pyfisch/servo,KiChjang/servo,ConnorGBrewster/servo,thiagopnts/servo,notriddle/servo,pyfisch/servo,larsbergstrom/servo,pyfisch/servo,saneyuki/servo,mbrubeck/servo,fiji-flo/servo,thiagopnts/servo,CJ8664/servo,thiagopnts/servo,larsbergstrom/servo,dati91/servo,canaltinova/servo,ConnorGBrewster/servo,nnethercote/servo,anthgur/servo,emilio/servo,rnestler/servo,pyfisch/servo,DominoTree/servo,larsbergstrom/servo,szeged/servo,mbrubeck/servo,jimberlage/servo,mbrubeck/servo,nnethercote/servo,eddyb/servo,dsandeephegde/servo,dsandeephegde/servo,SimonSapin/servo,pyfisch/servo,mattnenterprise/servo,sadmansk/servo,szeged/servo,peterjoel/servo,rnestler/servo,anthgur/servo,anthgur/servo,cbrewster/servo,ConnorGBrewster/servo,jimberlage/servo,canaltinova/servo,jimberlage/servo,mattnenterprise/servo,CJ8664/servo,anthgur/servo,mattnenterprise/servo,eddyb/servo,jimberlage/servo,saneyuki/servo,KiChjang/servo,anthgur/servo,DominoTree/servo,dsandeephegde/servo,mbrubeck/servo,rnestler/servo,fiji-flo/servo,nnethercote/servo,larsbergstrom/servo,mbrubeck/servo,upsuper/servo,dsandeephegde/servo,emilio/servo,pyfisch/servo,CJ8664/servo,saneyuki/servo,dati91/servo,DominoTree/servo,dsandeephegde/servo,KiChjang/servo,DominoTree/servo,szeged/servo,SimonSapin/servo,DominoTree/servo,emilio/servo,notriddle/servo,avadacatavra/servo,splav/servo,sadmansk/servo,cbrewster/servo,larsbergstrom/servo,splav/servo,szeged/servo,saneyuki/servo,szeged/servo,saneyuki/servo,notriddle/servo,cbrewster/servo,DominoTree/servo,szeged/servo,DominoTree/servo,DominoTree/servo,pyfisch/servo,eddyb/servo,avadacatavra/servo,jimberlage/servo,avadacatavra/servo,sadmansk/servo,nnethercote/servo,rnestler/servo